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LED experts, can you help me please?

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LED experts, can you help me please?

Old 01-27-09, 12:17 PM
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LED experts, can you help me please?

I wonder if someone with some expertise in electronics, specifically LED lighting, could tell me what's wrong with my headlight / taillight design.

Here's the problem in a nutshell: it works, but the tail light seems to give more light than the head light. What I'm wondering is: would I get more light from the headlight if I wired it differently, i.e. Cree stars in sequence instead of parallel; or would I get more light out of two Cree stars than three; or ... well, what else might I be missing? Obviously, my understanding of electronics is pretty basic, so feel free to point out the obvious-- it probably isn't obvious to me.

Okay, here are the details:

I first put the headlight together two weeks ago, and it was really bright, really dazzling. But within a half mile, my old tail light burned out. Probably not a coincidence, and not a surprise anyway. So last week I rebuilt the thing to include an LED tail light that I thought would be compatible. The result is mixed. The tail light is very bright, no problems there; but the head light is not. In fact it seems considerably less bright now than it was before I connected the tail light.

I used an aftermarket automotive foglight as a housing for the headlight, and an old Union bicycle taillight as the housing for the taillight; here they are as installed on the bike:

The AC comes from a Nexus HB-NX30 hub dynamo built into a 16" (305 mm ISO), and goes first
through a switch from Radio Shack (https://www.radioshack.com/product/in...ductId=2062505)
to a bridge rectifier from Radio Shack (https://www.radioshack.com/product/in...ductId=2062581),
to a 350mA constant current, non dimming buckpuck (https://www.ledsupply.com/03023-d-n-350.php).
Then to four LED's in parallel:
three Cree Q4 XR-E stars (https://www.ledsupply.com/creexre-w100.php) with appropriate optics (that's the headlight)
and one red K2 star (https://www.ledsupply.com/05027-pd12.php) with no optics at all.

Here's my attempt at a schematic:

As I understand it, the forward voltage drop of the XR-E's should be 3.3V each; and the red K2 should have a voltage drop of 2.95V; total 12.85V.

My dynamo, of course, is rated for only 6V, but that assumes a large (26" - 27") wheel; with my 16" wheel it puts out quite a bit more -- I've measured 15V at 10 mph (without load), which is more or less my minimum speed. I know it will flicker at low speed, that doesn't bother me. But at 15 mph I'm not getting as much light as I expected.

Any suggestions for possible fixes, or tests to diagnose possible problems, would be appreciated.

Thanks!
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Old 01-27-09, 12:35 PM
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Hi,

First thing you should do is throw away the buckpuck (LEDs will need to be in series) as the dynohub can generate enough voltage for 3 or 4 LEDs.

Secondly the red LED runs at a lower voltage so is going to be brighter (it will draw more current).

Thirdly add some smoothing.

Use one of the diagrams from here: https://www.pilom.com/BicycleElectron...moCircuits.htm

Finally, the buckpuck is doing nothing other than wasting power.


Last edited by Unknown Cyclist; 01-27-09 at 12:44 PM.
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Old 01-27-09, 12:54 PM
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Originally Posted by Unknown Cyclist View Post
Hi,

First thing you should do is throw away the buckpuck (LEDs will need to be in series) as the dynohub can generate enough voltage for 3 or 4 LEDs.
... the buckpuck is doing nothing other than wasting power.
Okay, thanks! But... with my small wheel, and doubled voltage, won't I run the risk of melting the LED's? I have melted quite a few already. And I do mean "melted"-- they start out really bright, but within a mile they're rattling around in the housing. I figured the buckpuck protects against that, no?
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Old 01-27-09, 01:08 PM
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Originally Posted by rhm View Post
Okay, thanks! But... with my small wheel, and doubled voltage, won't I run the risk of melting the LED's? I have melted quite a few already. And I do mean "melted"-- they start out really bright, but within a mile they're rattling around in the housing. I figured the buckpuck protects against that, no?
Why do you say the voltage is doubled ?

If the LEDs are in series, the highest the current can reach is appr. 500mA and the voltage will be related to the current.

You will still need a rectifier and smoothing.

And heatsinking.

How did you connect up the ones that melted ?
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Old 01-27-09, 02:12 PM
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Originally Posted by Unknown Cyclist View Post
Why do you say the voltage is doubled ?
As I understand it, a hub dynamo's voltage goes up proportional to teh bicycle's speed. My voltage is (almost) doubled because my wheel is small; it rotates a lot faster than the manufacturer intended. My small wheel makes about 105 revolutions per mile, while the dynamo is designed for a wheel that rotates something like 62 to 64 per mile. This is why some manufacturers make dynamos specially designed for small wheels; but they are much more expensive, and I don't have one of those.

Originally Posted by Unknown Cyclist View Post
If the LEDs are in series, the highest the current can reach is appr. 500mA and the voltage will be related to the current.
I'm glad to hear it, but I don't really understand. What's limiting the current to 500mA?

Originally Posted by Unknown Cyclist View Post
You will still need a rectifier and smoothing.

And heatsinking.
I understand the rectifier well enough, but not the smoothing. Frankly I'm not sure what smoothing is. How do I figure out how much smoothing I need? The heat sink I understand to some degree; but again, how do I figure out how much heatsinking I need?

Originally Posted by Unknown Cyclist View Post
How did you connect up the ones that melted ?
For the last two years I have used a home-made headlight using 5W, 12V halogen bulbs with, I think, an MR-11 base. The light was good, not great, but the system was durable; bulbs lasted about a year. At one point I bought two (rather expensive) LED bulbs that fit the same base, which gave a lot more light, but... well, the first one melted forthwith. Then I built a voltage regulator into the circuit, and... right, the second bulb melted too. I suspect I hadn't done the regulator right but, fed up, I went back to my halogen bulbs. At one point I had an off-the-shelf taillight with standlight function mounted to my mudguard, which was very nice, but... guess what? The leads to the LED melted.

Again: Thanks!
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Old 01-27-09, 02:24 PM
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I'm glad to hear it, but I don't really understand. What's limiting the current to 500mA?

The hub itself can't produce more than this. However atm your buckpuck limits the current to 350mA while your LEDs can handle a lot more than that.

I understand the rectifier well enough, but not the smoothing. Frankly I'm not sure what smoothing is. How do I figure out how much smoothing I need?

A big capacitor across the power to the LEDs (once you've removed the buckpuck) it will need to be connected the right way around.

The heat sink I understand to some degree; but again, how do I figure out how much heatsinking I need?

You may well find that heat is building up if the box is sealed. A large piece of aluminium is probably your best bet.

For the last two years I have used a home-made headlight using 5W, 12V halogen bulbs with, I think, an MR-11 base. The light was good, not great, but the system was durable; bulbs lasted about a year. At one point I bought two (rather expensive) LED bulbs that fit the same base, which gave a lot more light, but... well, the first one melted forthwith. Then I built a voltage regulator into the circuit, and... right, the second bulb melted too. I suspect I hadn't done the regulator right but, fed up, I went back to my halogen bulbs. At one point I had an off-the-shelf taillight with standlight function mounted to my mudguard, which was very nice, but... guess what? The leads to the LED melted.

When running LEDs the current is a function of the hub design and the voltage is a result of the LED characteristics.

Start again with a basic design using the LEDs that you have, some heatsinking a new rectifier with some smoothing and see how hot the lamp gets.

Have a look at this: https://www.pilom.com/BicycleElectronics/triLEDhead.htm

The LEDs are on an aluminium heatsink bolted into the back of the case - this is adequate heatsinking.

Again: Thanks!

I just hope you get it working !!
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Old 01-27-09, 05:12 PM
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I'd be interested to hear how well the front works if you just short out the taillight.
But yeah to get it going put all the leds in series and replace the buckpuck with a capacitor, maybe 1000uF 63V.
See pilom.com for how to do it properly.

Last edited by znomit; 01-27-09 at 05:18 PM. Reason: clarification
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Old 01-28-09, 10:30 AM
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Originally Posted by znomit View Post
I'd be interested to hear how well the front works if you just short out the taillight.
If the schematic is correct, then all the LEDs are in parallel, so shorting the taillight would mean shorting all the LEDs, and there should be no output at all.

Originally Posted by znomit View Post
But yeah to get it going put all the leds in series and replace the buckpuck with a capacitor, maybe 1000uF 63V.
See pilom.com for how to do it properly.
Yeah, the problem is that there is the same voltage across all the LEDs, so the whites are below their rated voltage, and the red is above its rated voltage. Series is the way to go.
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Old 01-28-09, 11:19 AM
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Originally Posted by twinquad View Post
Yeah, the problem is that there is the same voltage across all the LEDs, so the whites are below their rated voltage, and the red is above its rated voltage. Series is the way to go.
The driver will be limiting the total current to 350 mA.

The rear LED will be drawing about 150 mA, leaving about 200 mA for all 3 front LEDs.

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Old 01-29-09, 07:46 AM
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Originally Posted by twinquad View Post
Originally Posted by znomit View Post
I'd be interested to hear how well the front works if you just short out the taillight.
If the schematic is correct, then all the LEDs are in parallel, so shorting the taillight would mean shorting all the LEDs, and there should be no output at all.
Twinquad is right. I tried it, albeit accidentally (wire was pinched); the whole thing was dead.
Originally Posted by znomit View Post
But yeah to get it going put all the leds in series and replace the buckpuck with a capacitor, maybe 1000uF 63V.
1000uF, okay; but 63V? 6.3 seems to be a common capacitor rating, but I can't find any rated for 63. My actual voltage is in the range 12V- 15V; shouldn't I want a capacitor rated, I don't know, 16V? Where do I get it?
Originally Posted by znomit View Post
See pilom.com for how to do it properly.
Yes, I'm reading pretty much the whole site... but some of it is beyond my comprehension.
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Old 01-29-09, 11:03 AM
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Originally Posted by rhm View Post
Twinquad is right. I tried it, albeit accidentally (wire was pinched); the whole thing was dead.

1000uF, okay; but 63V? 6.3 seems to be a common capacitor rating, but I can't find any rated for 63. My actual voltage is in the range 12V- 15V; shouldn't I want a capacitor rated, I don't know, 16V? Where do I get it?

Yes, I'm reading pretty much the whole site... but some of it is beyond my comprehension.
16v should be ok (for up to 4 LEDs) and you should be able to get one from 'radioshack' afaik. I bought mine from Maplin, I'm in the UK.

The power lead from your rectifier should be +ve and -ve, the capacitor should be polarised and go across these two leads. ie the +ve of the capacitor goes to the +ve lead.

You only need to look at the first couple of circuits, which bits are you struggling with ?
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Old 01-29-09, 11:52 AM
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In your first post you link to 'LEDsupply' for the XRE Q4, on this site they have a link to the datasheet.

https://www.ledsupply.com/docs/cree-xre.pdf

Go to page 7 and look at the chart 'Electrical Characteristics'.

For 500mA the voltage will be about 3.3v - 3.4v.

Therefore if you are using 3 Q4s in series the max attainable voltage will be about 10 volts.

If you are using 4 Q4s in series the max attainable voltage will be about 13.6 volts.

A 16v capacitor should be fine.

When you buy some more LEDs buy R2s or whatever the latest LED is.

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Old 01-29-09, 12:23 PM
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Okay, do I have this right?




I didn't see a 16v capacitor at Radio Shack; would 35v work?
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Old 01-29-09, 12:33 PM
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Originally Posted by rhm View Post
Okay, do I have this right?

I didn't see a 16v capacitor at Radio Shack; would 35v work?
Looks good, though there might still be an issue with rear light brightness.

It might be worth replacing the bridge rectifier with Schottky diodes at some point in the future when you are happy it's all working properly.

With your hub spinning faster than it should you'll get max power early so the LEDs will be at 500mA more of the time and will get hot.

Heatsink !! Heatsink !! Heatsink !!

Last edited by Unknown Cyclist; 01-29-09 at 12:38 PM.
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Old 01-29-09, 01:42 PM
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Originally Posted by Unknown Cyclist View Post
Looks good, though there might still be an issue with rear light brightness.

It might be worth replacing the bridge rectifier with Schottky diodes at some point in the future when you are happy it's all working properly.

With your hub spinning faster than it should you'll get max power early so the LEDs will be at 500mA more of the time and will get hot.

Heatsink !! Heatsink !! Heatsink !!
Okay, will do! I'll let you know how it works.
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Old 01-29-09, 02:03 PM
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You might even want to consider putting a 5 ohm 3w resistor across the tail light. That would allow some of the current to bypass the rear tail light and will dim the tail light.
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Old 01-29-09, 02:18 PM
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Originally Posted by n0xdw View Post
You might even want to consider putting a 5 ohm 3w resistor across the tail light. That would allow some of the current to bypass the rear tail light and will dim the tail light.
The tail light won't be overly bright once the other LEDs are in series.

@ 500mA it will probably drop to 55 - 60 lumens.

See page 2 = https://www.ledsupply.com/docs/5027-Star-K2.pdf

However the white LEDS are likely to be a lot brighter @ 500mA



I would do away with the rear LED and fit a battery tail light, leaving more power for the front.
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Old 01-29-09, 02:34 PM
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Originally Posted by n0xdw View Post
You might even want to consider putting a 5 ohm 3w resistor across the tail light. That would allow some of the current to bypass the rear tail light and will dim the tail light.


Originally Posted by Unknown Cyclist View Post
...
I would do away with the rear LED and fit a battery tail light, leaving more power for the front.
Hmmm. Okay, I'll consider dimming the options after I've got the capacitor added &c. The head light is not really that important; something of a luxury. The tail light seems to me more critical to safety, and I'm rather enjoying its insane brightness. It seems to get car drivers' attention.

Thanks! More later!
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Old 01-29-09, 03:18 PM
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Originally Posted by rhm View Post
Hmmm. Okay, I'll consider dimming the options after I've got the capacitor added &c. The head light is not really that important; something of a luxury. The tail light seems to me more critical to safety, and I'm rather enjoying its insane brightness. It seems to get car drivers' attention.
Thanks! More later!
With your current set up your rear LED is getting maybe 150 mA so is about 28 lumens.

When wired correctly and moving fast enough to power the LEDs your rear LED should receive 500 mA which should be 55 - 60 lumens.

So it should be twice as bright, provided you are travelling fast enough.

It's rated for 75 lumens @ 700mA.

I doubt that you'll need to dim it.

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Old 01-29-09, 06:33 PM
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Originally Posted by rhm View Post
Hmmm. Okay, I'll consider dimming the options after I've got the capacitor added &c. The head light is not really that important; something of a luxury. The tail light seems to me more critical to safety, and I'm rather enjoying its insane brightness. It seems to get car drivers' attention.

Thanks! More later!
If the front light is just for being seen a single LED is fine. You will get much better low speed performance too. The equivalent of a single led is three in parallel so thats probably the best option given its built already... keep the three front leds in parallel and put the rear red in series.

Regarding caps Im sorry yes a 35V 1000uF will be fine, but I have a 35 and 63V sitting in front of me and they are the same size. I guess double the total Vf is a good rating, if you're not short on space higher V rating doesn't hurt.

When its all working we can talk about adding a standlight to keep the lights on when you stop.
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Old 01-29-09, 07:51 PM
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Originally Posted by znomit View Post
You will get much better low speed performance too. The equivalent of a single led is three in parallel so thats probably the best option given its built already... keep the three front leds in parallel and put the rear red in series.
Three LEDs in parallel WILL NOT give better low speed performance.

Three LEDs in parallel IS NOT the equivalent of a single LED.

The three LEDs would only get 1/3 of the current each and might not even light up, especially at low speed.

Edit: @ high speed each white LED should produce 20 - 25 lumens, but at low speed nothing

https://www.ledsupply.com/docs/cree-xre.pdf (page 7)

Sorry matey, that isn't good advice.


Last edited by Unknown Cyclist; 01-29-09 at 08:03 PM.
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Old 01-29-09, 09:32 PM
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I don't bother powering a taillight with the dynamo. A couple of AAA batteries last hundreds of hours and don't go out when you stop. You also don't need to run the wire to the rear. Battery operated taillights are cheap as well.
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Old 01-29-09, 09:59 PM
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Originally Posted by Unknown Cyclist View Post
Three LEDs in parallel WILL NOT give better low speed performance.

Three LEDs in parallel IS NOT the equivalent of a single LED.

The three LEDs would only get 1/3 of the current each and might not even light up, especially at low speed.
Circuit wise three in parallel behaves the same as one. The same voltage and total current, each led does get 1/3 the total. LEDs peak efficiency is around 20mA and light fine well under 10mA.
At low speeds they will do much better than three in series and a little better than a single led at moderate to high speeds. A four series LED light (three front plus one rear) will likely not light at 5kph where a two led light will put out 0.5 - 1W, which is heaps to be seen by.
For a commuter light the low speed performance is more important than putting out 500lm at 25kph
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Old 01-30-09, 02:55 AM
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Originally Posted by znomit View Post
A four series LED light (three front plus one rear) will likely not light at 5kph where a two led light will put out 0.5 - 1W, which is heaps to be seen by.
For a commuter light the low speed performance is more important than putting out 500lm at 25kph
Are you taking into account that his hub will be spinning twice as fast as it should be ?
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Old 01-30-09, 03:33 AM
  #25  
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Originally Posted by Unknown Cyclist View Post
Are you taking into account that his hub will be spinning twice as fast as it should be ?
Nope, still holds though, more light at lower speeds is the goal. That would be a good setup even when tootling through traffic near intersections (the only thing I don't like about my dyno lights).

BTW rhm that is the coolest front light I've seen, love the shape.
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